Process for coating metals



Patented Mar. 21, 1933 UNITED STATES PATENT; OFFICE GOODWIN H. HOWE, OFSCO'IIA, NEW YORK, ASSIGNOB 1'0 GENERAL ELECTRIC COMPANY, A CORPORATIONOI NEW YORK PROCESS FOR COATING METALS 1T0 Drawing. Application filedMay 29,

The present invention relates broadly to the protection of surfaces ofmetals which are subject to corrosion or oxidation. More specifically,this invention is concerned with an improved process for coating suchmetals with a protective layer of chromium.

It has been proposed heretofore to coat the surfaces of metals, eitherferrous or nonferrous, which are subject to corrosion or oxidation, withprotective metals such as chromium by bringing the coating metal with orwithout an inert medium, such as aluminum oxide, directly into contactwith the surface to be coated and then heating for a requisite period oftime at the proper temperature. While this method is successful ineffecting a coating of protective metal on the surface of the metalcoated, its practice is, nevertheless, limited in certain cases. Forexample, where objects of varied or complex shape have been involved, alarge amount of material has been required to be packed around theobject in order to insure complete coating of all the surfaces thereof.Again, the objects must always be packed with more or less care so thatthe surfaces may receive the proper coating of protective metal.Moreover, it is often inconvenient to handle large amounts of thispacking material in coating objects by this method.

It has been recognized that coating with a protective metal in the vaporphase not only facilitates the operation and makes possible andpractical the coating of objects with a minimum amount of coating material, but also produces a better coating of protective metal. Thecoating in the vapor phase with metals of zinc and aluminum has 4 beenaccomplished heretofore with compara-v tively little difliculty inasmuchas relatively low temperatures could be used in effecting the operation.For example, while zinc melts at about 419 C. and boils at about 45 930C. its vapor pressure is such that at about 400 C. coating in the vaporphase may be-successfully carried out. Again in the case of aluminumwhich melts in the neighborhood of 660 0., vapor phase coating 5is'found practical at about 950 G. How ever, with chromium as thecoating metal 1930. Serial No. 457,482.

no such low temperatures can be used in carrying out the coatingoperation in the vapor phase. Chromium melts in the vicinity of 1615 C.and in order to effect a coating with this metal in the vapor phase itrequires a temperature in the neighborhood of 1500 to 1600 C., andheating at this temperature for a prolonged period of time. Obviously,such high temperatures are not practical for commercial application.

I have discovered a practical method whereby I can protect the surfacesof a metal subject to corrosion or oxidation (hereinafter termed thefoundation metal), using chromium and effecting the coating in the vaporphase at a temperature considerably lower and in a much shorter periodthan has heretofore been possible and obtain a smooth, adherent,resistant coatmg of chromium.

According to my invention, the article to be protected is placed in aporous receptacle such as a porous alundum tube, which is then closed atits open ends and within which has also been placed but out of contactwith the articl.e,a relatively small amountof chromium. This receptacleis placed in another suitable container such as an iron tube and packedwith an inert material such as aluminum oxide and containing a smallamount of a carrier. The function of the carrier will 8 be more fullypointed out hereinafter. If desired, some chromium may be added to theinert material but this is not necessary. The container so packed isthen placed in a furnace and heated for the requisite period of time,the atmosphere of the furnace being either air or an inert or reducingatmosphere, if desired, although the latter are not necessary to thesuccessful operation of the process. The chromium deposits on thearticle in the form of a smooth, adherent, resistant protective coatingand is ver similar to a coating of chromium obtained yelectro-deposition.

As illustrative of the process of my invention, but in no way limitingit, the following is given:

A piece of iron in the form of a wire or rod, the exact shape being ofno material consequence, is placed in a porous alundum containertogetherwith a small amount of chromium, preferably a few small piecesof chromium. The chromium is at one side of and out of contact with theiron. The alundum container which may be in the form of a piece oftubing, is then plugged at its open ends with asbestos paper. Itis thenplaced in an iron container such as an iron tube and completelysurrounded by an inert medium. This inert medium may consist of suchmaterial as aluminum oxide to which may be added, if desired. somechromium. Mixed with this inert material is a small percentage of acarrier. As an example of the carrier which I employ ammonium chlorideis cited. A com'enient mixture to use for the inert medium comprisesapproximately 49% aluminum oxide, 50% powdered chromium, and 1% ammoniumchloride. The iron tube packed with this mixture is placed in a cold airfurnace and heated to about 1000 C. for about two hours. After thistreatment the iron rod has a bright, resistant coating of chromiumthereon.

The mixture outside of the alundum tube may consist entirely of an inertmaterial, such as the aluminum oxide, with the carrier and the sameresults will be obtained as above. The percentage of carrier which maybe used may be varied although I have found'that up to approximately 2%by weight of carrier yields the best results, Results with about 5% byweight of the carrier are not satisfactory. The temperature at which thecoating takes place may be varied within the approximate limits 950 to1100 C., although I have found that about 1000 C. is the optimumtemperature. The time of heating is considerably shorter than thatnecessary where the coating is packed around the object to be coated andin general depends on the'size of the object to be protected. Two hoursordinarily gives a good protective layer; and there is usually no objectin prolonging the heating much beyond this period as the carrier isusually spent by this time.

\Vhile I do not wish to be limited to the following theory, the bestexplanation I now have as to the mechanism of the process is as follows:

Assuming that ammonium chloride is used as the carrier for the chromiumduring heating the ammonium chloride breaks down into ammonia andhydrochloric acid, the latter combining with the metallic chromium toyield chromium chloride which in turn reacts with the foundation metal,iron, for example, the chromium depositing thereon in the form of asmooth adherent, resistant, hard coating. The reactions take place inthe vapor phase, the ammonium chloride acting as the carrier for thechromium.

It is evident that the present process gives a convenient and practicalway of protecting metallic surfacesagainst corrosion or oxidation, usingchromium as the protective metal. The temperature at which the processmay be carried out is considerably lower than when no carrier'is usedfor the chromium, and the time of heating is materially shortened. Amuch smoother coating of protective metal is obtained by means of thepresent invention than is obtained by the use of prior processesinvolving the packing of the coating metal directly on the objectcoated.

Vhat I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. The method of providing a foundation metal with a protective coatingof chromium, which comprises heating the said metal in the presence of,but out of contact with, chromium and with a carrier which is out ofcontact with both the foundation metal and the chromium.

2. The method of providing a foundation metal with a protective coatingof chromium, which comprises heating the said metal in the presence of,but out of contact with, chromium and with ammonium chloride'which isout of contact with both the foundation metal and the chromium.

3. The method of providing a foundation metal with a protective coatingof chromium which comprises heating said metal in the presence of, butout of contact with, chromium and with ammonium chloride which is out ofcontact with both the foundation metal and the chromium, at about 1000C. until the coating results.

4. The method of providing a foundation metal with a protective coatingof chromium, which comprises heating said metal in the presence of, butout of contact with, chromium and with about 2% by weight of a carrierwhich is out of contact with both the foundation metal and the chromium.

5. The method of providing a foundation metal with a protective coatingof chromium, which comprises heating the said metal in the presence of,but out of contact with, chromium at a temperature of about 1000 C. andwith a carrier which is out of contact with both the foundation metaland the chromium, said carrier consisting of not more than about 2% byWeight of ammonium chloride.

6. The method of providing a foundation metal with a'protective coatingof chromium which comprises placing said metal and chromium within aporous receptacle out of contact with each other, packing theexterior ofsaid receptacle with an inert medium containing a small amount ofcarrier therein and heating to form a coating on said metal.

7. The method of providing a foundation metal with a protective coatingof chromium which comprises placing said metal and chromium within aporous receptacle out of contact with each other, packing the exteriorof said receptacle with an inert medium containing a small amount ofammonium chlo- Q ride therein and heating to form a coating on saidmetal.

8. The method of providing a corrodible foundation metal Wit aprotective coating of chromium which comprises placing said metal in arefractory porous receptacle, placing a small amount of chromium in saidreceptacle out of contact with said metal, placing said receptacle in ametal container and surrounding said receptacle within the metalcontainer with an inert medium com rising a small amount of ammoniumchlori e, and heating said receptacle andcontainer to a temperature ofabout 1000 C. to produce a protective coating of chromium on said metal.In witness whereof I have hereunto set my hand this 28th day of May,1930.

GOODWIN H. HOWE.

